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1 Molecular Geometry

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2 Molecular Structure Molecular geometry is the general shape of a molecule or the arrangement of atoms in three dimensional space. Physical and chemical properties depend on the geometry of a molecule.

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3 3-D Molecular Structures 3-D Model 3-D Drawing

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4 Does it matter? The Thalidomide Story The chemical structure of thalidomide. “The Same and Not the Same”, by Roald Hoffmann 1995, Columbia University Press models – enantiomers (mirror image)

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5 Does it matter? Fatty Acids ‘trans’ fatty acid ‘cis’ fatty acid

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6 VSEPR Model The Valence Shell Electron Pair Repulsion model predicts the shapes of molecules and ions by assuming that the valence shell electron pairs are arranged as far from one another as possible to minimize the repulsion between them.

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7 AXE shorthand notation: A - central atom X - substituent atoms E - lone pair electrons AX 3 VSEPR Notation

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8 Only five basic shapes. When a lone pair replaces an atom, the molecular geometry changes as well as the angles. Double and triple bonds are treated as single bonds. Predicting Molecular Geometry: VSEPR

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9 Linear bond loneMolecular pairs pairsGeometry 2 0 linear.. 1 1-3 linear

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10 Geometry is Linear. Bond angle is 180 o. AX 2 Example 1: BeCl 2 Cl Be Cl 1. Draw the Lewis structure 2. Two electron pairs around the central atom. Two substituents Zero lone pairs. Predicting Molecular Geometry

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11 Predicting Molecular Geometry Example 2: HF 1. Draw the Lewis structure 2. One electron pair between the two atoms. Three lone pairs on fluorine. Geometry is Linear. AX

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12 bond loneMolecular pairs pairsGeometry 3 0 trigonal planar Trigonal Planar Three e- pairs about central atom

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13 Predicting Molecular Geometry trigonal planar AX 3.. F: :F B :F:.. Example 3: BF 3 Three electron pairs around the central atom. Three substituents and Zero lone pairs.

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14 Tetrahedral bond loneMolecular pairs pairsGeometry 4 0 tetrahedral Four e- pairs about central atom

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15 Predicting Molecular Geometry CH 4 AX 4 Example 4: H H H C H Four electron pairs around the central atom. Four substituents and Zero lone pairs. tetrahedral

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Trigonal Pyramidal 16 3 1 trigonal pyramidal.. bond loneMolecular pairs pairsGeometry Four e- pairs about central atom

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17 Predicting Molecular Geometry NH 3 AX 4 :AX 3 molecular geometry = triangular pyramidal Example 5: H H N H H N H H Four electron pairs around the central atom. Three substituents and One lone pair.

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18.. 2 2 bent Bent bond loneMolecular pairs pairsGeometry Four e- pairs about central atom

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19 Predicting Molecular Geometry H2OH2O AX 4 :AX 2 molecular geometry = angular or bent Example 6: H O H Four electron pairs around the central atom. Two substituents and Two lone pairs. O H H

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20 Predicting Molecular Geometry Tetrahedral - bond angles Order of increasing repulsion : bonding pair-bonding pair < bonding pair-lone pair < lone pair-lone pair

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